On possession call paid remaining amount, as per agreement and builder asking for more money

Hello sir, I have purchased a flat in panvel, navi mumabi in August 2020. That time as per RERA possession date was Dec'2021. but verbally sales manager was committed to give the possession in march'21. After that possession date was extended twice and that date is now Dec'22. Agreement and registration of the flat done in Dec'2020. I have taken loan from my employer (company where i am working). on 5th aug'21, i have received a demand letter from builder including delay charges after that i have talked to the builder office because some 10% of the loan amount was supposed to disbursed in june'21. Loan and advance section of my company was asking for progress report along with architect certificates and the same was not provided by builder on time (i have mail and whatsapp chat record regarding this). On 7th august 2021, builder provided the architect certificate and a revised demand letter on 9th aug'21 without delay charge. My company where i am working hold the 5% amount of the loan and they said it will be released after OC received. in july 2022, builder has received OC in july 22 and the remaining loan amount has been paid. Now in September,22 when i have asked for final possession they have called me in their office and asked me for more amount of Rs. 6L, saying you have due amount delayed charges for payment. after given the all proof and discussed they said ok pl pay the remaining amount of 10 lakhs (mentioned in agreement that it will be paid on possession) along with 50K GST i have paid to the builder, after signed some paper for maintenance and society security charges (50K). they asked us to come and collect key of the flat. the very next day i came for my duty (working outside of mumbai) when my wife asked them about key then builders official is saying you have to pay your due amount Rs. 6L . Now there are asking for at least Rs 3.5 L and 18 % of GST. till date the builder did not send me or i did not received any legal notice regarding delay charges. i have strong proof - whatsapp chat with builder officials, mail record, i have paid all money by bank transfer. pl. suggest what to do, so that i can receive my possession letter along with flat key. Regards,